Prepare a 6 inch spring foam pan or a pan with removable base. Alternately you may use a regular cake pan lined with parchment paper.
Mix together sugar, cocoa & milk in a pan. Whisk well to make a smooth liquid, breaking up lumps of cocoa if any.
Add butter (optional) & boil this stirring often until you get a slightly thick syrup.
To test, dip a spoon and check. The syrup must coat the back of the spoon.
This will further thicken upon cooling. Set aside to cool down.
Break down the biscuits approximately to 1 by ½ inch pieces.
When the syrup cools down pour it in 2 batches and give a good stir.
Transfer the chocolate biscuits to the pan and press down with the base of a measuring cup. Smoothen the top with a spatula.
Cover it and refrigerate for 1 to 2 hours, until set.
Add the chocolate and milk to a small bowl.
Microwave it for 1 minute, stirring every 30 seconds until the chocolate melts and turns smooth.
Or Double boiler - Alternately place the bowl over a small pot of hot water. Stir and mix until smooth.
Unmold the cake gently from the pan. Or lift the cake from the pan holding the butter paper.
Spread the chocolate frosting over the biscuit cake with a spatula. Smear some chocolate on the sides & smoothen.
Refrigerate until the chocolate frosting is set. Slice the biscuit cake and serve plain or with Ice cream or tea.